High quality woodworking with safe operation requires the ability to maintain very close tolerances. Woodworking power tools such as table saws are known to include a straight groove or slot, known as a miter slot or a T-slot (referring to its common cross-sectional shape), for slidably receiving a guide bar. The guide bar moves within the T-slot to guide a work piece through a cutting tool, such as a saw blade, in a precise linear direction parallel to a desired cut line. Any gap between the width of the guide bar and the width of the slot allows for some undesirable horizontal and angular movement of the guide bar and the guided work piece as it is moved through the cutting tool.
The width of a T-slot has a nominal dimension, for example 750 mils on many table saws, but that dimension may vary by as much as 2-10 mils for any given table due to manufacturing tolerances. Such variation can create an unacceptable amount of positional uncertainty during woodworking operations. There is an ongoing effort in the woodworking industry to develop guide bars that can accommodate such variations in the slot width in order to minimize any gap between the width of the guide bar and the width of the slot. For example:
U.S. Pat. No. 5,097,601 issued on 24 Mar. 1992 describes the use of a plurality of set screws that can be rotated to project from the sides of the guide bar to span the gap between the guide bar and the slot, thereby ensuring a tight fit between the guide bar and the slot. Adjustment of the set screws prior to inserting the device into the slot requires a trial and error approach, or alternatively, adjustment of the set screws when the device is in the slot is difficult due to their horizontal orientation and limited access to the set screws.
U.S. Pat. No. 5,275,074 issued on 4 Jan. 1994 describes the use of a plurality of wedging members to spread apart sections of the guide bar side walls to ensure engagement with the adjoining slot walls. The wedging members are controlled by screws which can be accessed conveniently from above the slot. However, localized deformation of the guide bar side walls does not provide a uniform fit of the guide bar along the full length of the guide bar within the slot.
U.S. Pat. No. 5,402,581 issued on 4 Apr. 1995 and U.S. Pat. No. 6,502,492 issued 7 Jan. 2003 describe two-piece guide bar assembly designs wherein top and bottom members can be horizontally displaced relative to each other to expand the overall width of the device to obtain a uniform fit within the slot. In the '581 patent, the top and bottom members are urged apart by small springs to expand the device within the slot.